The disclosed technology relates to a plastic compound suitable for preparing articles, such as pipe fittings and valves, with good physical properties, such as impact strength, and resistance to environmental stress cracking (ESC). In particular, the technology relates to a vinyl chloride resin, which includes chlorinated polyvinyl chloride (“CPVC”) homopolymer. Furthermore, the invention relates to vinyl chloride homopolymer compounds containing the vinyl chloride homopolymer resin, and articles made from such compounds, which meet 23447 cell classifications under ASTM D1784.
Polyvinyl chloride (PVC) is a vinyl chloride polymer having about 57 mol % chlorine bound along a polymerized ethylene backbone. Chlorinated polyvinyl chloride (CPVC) is a post-chlorinated form of PVC typically having greater than 57 mol % bound chlorine. CPVC is known to have excellent high temperature performance characteristics, among other desirable physical properties.
CPVC is an important specialty polymer due to its high glass transition temperature, high heat deflection temperature, outstanding flame and smoke properties and chemical inertness. While the glass transition temperature of the CPVC generally increases as the amount of chlorine increases, increased chlorine content causes the CPVC to become more difficult to process and products made therefrom to become more brittle. In this regard, it is known that CPVC resins generally have low impact properties and often require compounding with impact modifiers.
It is also known that CPVC resins are subject to environmental stress cracking. Many polymeric materials, loaded mechanically and immersed in certain kinds of liquids, undergo failures by crazing and/or cracking. The loads required are much less than those required of failures in air. The failure promoting liquids are non-solvents and chemically inert for polymers. Failures like these are called environmental stress crazing (ESCR), environmental stress cracking (ESC), and environmental stress failure (ESF) which includes both.
In simple terms, ESC occurs from an external or internal crack in a plastic caused by tensile stresses less than the plastic's short term mechanical strength, resulting in failure. The addition of an organic liquid (the environment) with the applied stress can result in ESC failures. In essence, the organic liquid wets the surface of the polymer and in combination with the tensile stress, accelerates the failure rate. This phenomenon was identified as far back as the 1940s in the general thermoplastics field.
Currently in the field of CPVC pipe fittings, recommendations are in place on construction practices that will limit contact of the CPVC pipe fitting with incompatible materials. However, a more fundamental approach for improving the resistance to ESC for CPVC articles is desired.
A CPVC resin that can be readily processed and that can be employed in a CPVC compound to produce a final product having improved resistance to environmental stress cracking, with at least maintained or improved impact strength over traditional CPVC compounds, and meeting cell class 23447 under ASTM D1784 would be desirable.